1. Field of the Invention
The present invention relates to a driving method of a display device for performing a display by a time gray scale method.
2. Description of the Related Art
As a driving method of a light emitting device that is one of display devices, there is known a time gray scale method in which a light emission period of pixels in one frame period is controlled with binary voltage of a digital video signal to display a gray scale. Electroluminescent materials are more suitable for a time gray scale method than liquid crystals and the like since the response speed is generally faster. Specifically, when performing display by the time gray scale method, one frame period is divided into a plurality of subframe periods. Then, a light emitting element in a pixel emits light or not in accordance with a video signal in each subframe period. According to the aforementioned structure, the total actual light emission period of the pixels in one frame period can be controlled by a video signal, so that a gray scale can be displayed.
However, in the case of performing display using the time gray scale method, there is a problem in that a pseudo contour may be displayed in a pixel portion depending on the frame frequency. Pseudo contours are unnatural contour lines that are often perceived when the middle gray scale is displayed by the time gray scale method, which is considered to be mainly caused by a variation of the perceptual luminance due to a characteristic of human visual sense.
As a technique to prevent the above-described pseudo contour, a driving method of a plasma display has been proposed in which a subframe period for light emission appears continuously within one frame period (see Patent Document 1). According to the driving method, such a phenomenon that a light emission period and a non light emission period within each frame period are inverted in adjacent frame periods can be prevented, so that generation of pseudo contour can be suppressed.
[Patent Document 1] Japanese Patent Laid-Open No. 2000-231362 (p. 5, paragraph 0023)
However, in the driving method disclosed in Patent Document 1, the total gray scale level equals to the number of subframe periods for one frame period. Therefore, when the number of subframe periods is increased in order to increase the total gray scale level, each subframe period is required to be shortened. However, video signal input to pixels of all rows is generally required in each subframe period. Thus, in the case where the subframe period is too short, the operating frequency of a driver circuit is required to be increased. Considering the reliability of a driver circuit, it is not preferable to make a subframe period shorter than is necessary.
Note that each subframe period can be lengthened to some extent by lengthening a frame period. However, lengthening the frame period is not preferable in that drastic increase of the total gray scale level is not to be realized, besides, a flicker is to be more easily generated.
In Patent Document 1, therefore, a technique for increasing the total gray scale level to be displayed in a pseudo manner without increasing the number of subframe periods is also described, in which image processing such as dithering is performed. However, by performing the image processing such as dithering, the total gray scale level to be displayed can be increased while the image is displayed as if sand is spread thereover, leading inevitably to decrease in image quality.